One of known oil supplying apparatuses for a vehicle is a lubrication oil supplying apparatus for an engine. The lubrication oil supplying apparatus includes a supply passage to a crank system for supplying lubrication oil discharged from an oil pump, which is a fixed displacement oil pump, driven by the engine to a main bearing of a crank shaft. The lubrication oil supplying apparatus for the engine also includes a supply passage to a valve train for supplying the lubrication oil discharged from the oil pump to lubricating portions of the valve train. In the lubrication oil supplying apparatus for the engine, an orifice for the crank system and an orifice for the valve train are provided on the supply passage to the crank system and the supply passage to the valve train respectively for varying resistance of oil flow in the passages. A control system controls the resistance of the orifice for the crank system to decrease and the resistance of the orifice for the valve train to increase as an engine speed increases (refer to paragraphs 0007 to 0013 and FIG. 1 of JP6-2112932A, herein after referred to as Reference 1). According to the lubrication oil supplying apparatus for the engine, the resistance of the orifice for the crank shaft is increased and the resistance of the orifice for the valve train is decreased at a low engine speed, thereby ensuring a sufficient amount of oil flow to the lubricating portions while reducing a discharge amount of the oil pump. At a high engine speed, the resistance of the orifice for the crank shaft is decreased and the resistance of the orifice for the valve train is increased, thereby ensuring a sufficient amount of oil flow to the main bearing of the crank shaft while reducing the discharge amount of the oil pump. At the same time, an amount of oil supplied to the lubricating portions of the valve train via the supply passage to the valve train is prevented from being excessive, and thus an amount of oil returning from the lubricating portions of the valve train to an oil pan is controlled not to exceed a collecting capacity of a passage that returns the oil to the oil pan.
In a similar oil supplying apparatus for a vehicle, oil is discharged from an oil pump driven by an engine to a main passage, and then supplied to lubricating portions of a crank system via a branch passage to the crank system and to lubricating portions of a valve train via a branch passage to the valve train. According to the oil supplying apparatus for the vehicle, a variable displacement oil pump is used. The variable displacement oil pump reduces a discharge amount thereof at or above a predetermined discharge pressure. The oil supplying apparatus for the vehicle also includes a fluid pressure sensing variable throttle provided on the branch passage to the valve train for increasing resistance of oil flow by throttling the oil flow as fluid pressure increases, or an oil temperature sensing variable throttle provided on the main passage for decreasing the resistance of oil flow by widening the oil flow as an oil temperature increases (refer to paragraphs 0006 to 0014 and FIG. 1 of JP2002-303111A, hereinafter referred to as Reference 2). A case where the fluid pressure sensing variable throttle is provided on the branch passage to the valve train is described below. When the discharge pressure rises as a discharge amount of the oil pump is increased by an increased engine speed, the resistance of the oil flow in the branch passage to the valve train is increased to prevent more lubrication oil from being supplied to the branch passage to the valve train than is needed. Consequently, a greater amount of lubrication oil is supplied to the branch passage to the crank system. As a result, the variable displacement oil pump is applied to supply an appropriate supply amount of the lubrication oil to the branch passage to the crank system. A case where the oil temperature sensing variable throttle is provided on the main passage will be describe below. At a low oil temperature, when the discharge amount of the oil pump is increased by the increased engine speed, the discharge pressure of the oil pump increases more rapidly compared to at a higher oil temperature. Consequently, at the low oil temperature, the discharge pressure of the oil pump reaches a predetermined discharge pressure (a pressure at which the oil temperature sensing variable throttle starts functioning) when the engine speed is lower, compared to at the high oil temperature. After the predetermined discharge pressure is reached, the supply of the lubrication oil to the main passage is reduced.
One of known engine oil supplying apparatuses is provided not only with a mechanical oil pump but also with an electric oil pump assisting the mechanical oil pump. In the engine oil supplying apparatus, a discharge port of the mechanical oil pump and an intake port of the electric oil pump are coupled to each other to parallelly connect the mechanical oil pump and the electrical oil pump. The engine oil supplying apparatus includes a first relief valve that opens when a fluid pressure of the discharge port of the mechanical oil pump is equal to or greater than a first predetermined fluid pressure so as to control the fluid pressure of the discharge port of the mechanical oil pump at or under the first predetermined fluid pressure, and a check valve connected to both ends of the electric oil pump for allowing oil to flow from the intake port to a discharge port of the electric oil pump. The engine oil supplying apparatus also includes a second relief valve provided between the discharge port of the electric oil pump and an oil jet for opening when a fluid pressure of the discharge port of the electric oil pump is equal to or greater than a second predetermined fluid pressure so as to allow the oil to flow from the discharge port of the electric oil pump to the oil jet (refer to paragraphs 0011 to 0013 and FIG. 1 of JP2004-116430A, hereinafter referred to as Reference 3). According to the engine oil supplying apparatus, the discharge port of the mechanical oil pump is connected to a lubrication passage for supplying lubrication oil to each part of the engine, the discharge port of the electric oil pump is connected to a variable valve timing system (valve timing control device), and the second predetermined fluid pressure is set to be higher than the first predetermined fluid pressure. Consequently, the variable valve timing system of the engine oil supplying apparatus is operated by operating the electric oil pump when an engine speed is low, and the electric oil pump is stopped when the engine speed is high. In addition, when the engine speed is high, the electric oil pump is actuated to operate the oil jet.
According to the oil supplying apparatuses disclosed in References 1 and 2, an amount of supplied oil by the oil pump is considered in controlling the fluid pressure but pressure of the supplied oil is not considered because the oil is supplied from the oil pump for lubrication purpose. Consequently, when a hydraulic actuator or similar system is provided on a hydraulic circuit of the aforementioned oil supplying apparatus, problems including delay in operation of the hydraulic actuator may occur. In particular, it is unavoidable that the operation of the hydraulic actuator is unstable due to insufficient fluid pressure when the engine speed is not high enough, for example, at engine start, because the mechanical oil pump is driven by the engine. According to the engine oil supplying apparatus disclosed in Reference 3, the mechanical oil pump and the electric oil pump assisting the mechanical oil pump are serially arranged, so that the electric oil pump compensates the insufficient fluid pressure of the mechanical oil pump. However, application of the electric oil pump for assisting the mechanical oil pump causes increased cost and weight, and a tight installation space.
A need thus exists for an oil supplying apparatus for a vehicle, which is not susceptible to the drawback mentioned above.